Borohydrides reduction method

The consumption of sodium borohydride (reduction method) upon reduction of carbonyl groups can also be used for their quantification. Remaining NaBH4 is quantified by the amount of hydrogen formed after reaction with acid [86,87]. 

The ternary catalyst systems are developed in order to illustrate the concept synergy towards the oxidation of alcohols having more than one carbon. Dutta et al. [66] synthesized Pt-Pd-Au/C catalyst using the borohydride reduction method, and... 

Froblems associated with borohydride reduction include its irreproducibil-ity, specially in the aqueous medium and the incorporation of boron in the product [158,159]. Bonneman and coworkers [160,161] suggested the use of tri-ethylborohydrides to avoid the incorporation of boron. In order to demonstrate the versatility of the borohydride reduction method, Bonneman and coworkers reduced a range of metal halides of early transition metals such as Ti, Zr, V,... 

The erythro aminoalcohols which were one of the most important key-intermediates were synthesized stereoselectively by the catalytic reduction of the corresponding hydroxyiminoketones which were derived from the propiophenones (5) (Scheme 2). Alternatively the erythro aminoalcohols were synthesized by the sodium borohydride reduction of the aminoketones which were obtained by the Gabriel reaction or the Neber rearrangement or some other well-known methods (7-9). The sodium borohydride reduction method can be applied for... 

An interesting quaternary catalyst, namely PtRuRhNi was proposed by Park et al. [125]. Rh has been considered early on for CH3OH oxidation as a co-catalyst with Pt in binary PtRh formulations, with maximum acitivity (compared to pure Pt) observed at low Rh loadings of —10 at% [126]. In the quaternary composition prepared by the borohydride reduction method, low Rh and Ni content was employed eorresponding to Pt Ru Rh Ni = 10 8 1 1 atomic ratios [125]. In DMFC experiments using 5 mg cm" anode eatalyst load a maximum power output at 343 K of 180 mW em was achieved with PtRuRhNi eompared to 160 mW cm" obtained with PtRu. Moreover, the quaternary eatalyst showed very good stability over an extended operation time of 20 h [126],... 

Finally in this section on deracemization via cyclic oxidation/reduction methods, there has been some limited work carried out on the deracemization of secondary alcohols. Soda et al. [22] employed lactate oxidase in combination with sodium borohydride to deracemize D/i-lactate (18) via the intermediate pyruvate (19) (Figure 5.12). 

Sodium borohydride reductions of gold(I) complexes give Au clusters at RT if sodium borohydride in ethanol is dropped slowly into a suspension of the Au(I) complex in the same solvent. The immediate coloring of the reaction mixture (mostly red), even after only a few drops of the borohydride have been added, indicates fast formation of Au clusters. In view of the complicated composition of these compounds the fast formation is surprising. The use of H2 and CO with HjO as reducing agents in the synthesis of gold clusters has been described (see Table 1, Method A, 8.2.2.2). 

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

The Bischler-Napieralski reaction is one of the traditional methods for isoquinoline synthesis, and has been applied to the preparation of fused quinolizidine systems. One simple example is the transformation of compound 246 into a 9 1 mixture of diastereomers 247 and 248 by treatment with phosphorus oxychloride followed by sodium borohydride reduction of a nonisolated iminium salt resulting from the cyclization (Scheme 49) <2000BMC2113>. 

Brinckmann [570] generated calibration curves by the P/T gas chromatog-raphy-FPD method for borohydride reductions of SnIV, Sn11, and Me2Sn2+ species to SnH4, SnI [8, or SnMe2H2, respectively, in distilled water, 0.2 M sodium chloride, and bay water. All three analytes showed a substantial in-... 

Ketone 166, the key intermediate of deplancheine was obtained in 63% overall yield by the reaction of harmalane (150) with l-bromo-2,3-epoxypropane, followed by sodium borohydride reduction and Moffat oxidation (116). Methods for the elaboration of the exocyclic, -configurated double bond are reviewed (117). 

Many of the methods used for the preparation of mononuclear hydrides may be applied to the polynuclear systems. Base attack on metal carbonyls, which furnished one of the first methods for the production of carbonyl hydride species, is applicable to a wide range of carbonyls. Borohydride reduction leads to a variety of products, depending upon the reaction conditions, Os3(CO)12 reacting with NaBH in di-oxane under reflux to give, after 4 hours, a mixture of the anions [H30s4(C0)12] and [H2Os4(CO)i2]2 (79). The related reaction in tetra-hydrofuran for 1 hour yields the anion [HOs3(CO)n]- as the main product with minor amounts of the two tetranuclear anions. 

The borohydride reduction-periodate cleavage applied to 2,3-O-isopro-pylidene-D-ribono- 1,4-lactone (16a) led to L-erythrose (30). The method was also employed (31) for the synthesis of D-erythrose, starting from an Obenzylidene-D-ribonolactone. However, in this case, the structural assignments for the intermediate compounds must be revised, as the starting material formulated as 3,5-O-benzylidene-D-ribono-1,4-lactone (2) was, as discussed previously in this section, the 3,4-0-benzylidene-D-ribono-1,5-lactone (3a). Therefore, the correct structure for the product described as 3,5-O-benzylidene-D-ribitol (20, not isolated) would be 3,4-O-benzylidene-... 

The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentration in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydrideforming elements exist in two oxidation states which are not equally amenable to borohydride reduction. For example, potassium iodide is often used to pre-reduce AsV and SbV to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se IV to elemental selenium from which no hydride is formed. For this and other reasons Thompson et al. [132] found it necessary to develop a separate procedure for the determination of selenium in soils and sediments although arsenic, antimony and bismuth could be determined simultaneously [133]. A method for simultaneous determination of As III, Sb III and Se IV has been reported in which the problem of reduction of Se IV to Se O by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride [134], Goulden et al. [123] have reported the simultaneous determination of arsenic, antimony, selenium, tin and bismuth, but it appears that in this case the generation of arsine and stibene occurs from the 5+ oxidation state. 

Radical addition of dibromodifluoromethane to alkenes followed by sodium borohydride reduction is a convenient two-step method for the introduction of the difluoromethyl group.5 Either one or both carbon-bromine bonds in the intermediate dibromides may be reduced, depending on the reaction conditions. In the case of acyclic dibromodifluoromethane-alkene adducts, the reduction occurs regioselectively to yield the relatively inaccessible bromodifluoromethyl-substituted alkanes. The latter are potential building blocks for other fluorinated compounds. For example, they may be dehydrohalogenated to 1,1-difluoroalkenes an example of this methodology is illustrated in this synthesis of (3,3-difluoroallyl)trimethylsilane. 

Methylation analysis was run by the method of Hakomori ( ), followed by hydrolysis with trifluoracetic acid, sodium borohydride reduction, and acetylation. 6LC was performed on a Hewlett-Packard 5970, used as an inlet for a mass spectrometer. Molecular weight was determined on a Sephacryl S-500 column (2.6 x 70 cm), using deionized water as solvent, upward flow, 2.75 ml/min, and detection by refractive index monitor. Model R-401 (Waters Associates). 

In analogy to 23, the chiralities of [2.2]meta- and [10]paracyclophanecarboxylic acids were also deduced from the results of kinetic resolutions 40-77>. For the application of Horeau s method, (—)-[10]paracyclophanecarboxylic acid (14) was transformed by stereoselective hydrogenation and subsequent sodium borohydride reduction of an intermediate cyclohexanone into the (—)-cis-cyclohexanol 94 which on reaction with racemic 2-phenylbutanoic anhydride afforded a 15% excess of the Ievorotatory acid thereby proving (in agreement with the kinetic resolution of the anhydride of 14, vide supra) the chirality (5) for (—)-14 and all its derivatives 40). Optical comparison with dioxa[10]paracyclophanecarboxylic acid (16) confirmed this result63,108). 

Diastereoselective reduction by chemical methods For the preparation of both enantiomers of dihydroxy ester sy -5a Prasad s [19] sy -selective borohydride reduction was applied, giving dihydroxy esters syn-5a in a diastereomeric ratio of dFsy a ti = 28 1 to 45 1. The enantiomers of dihydroxy ester anti-Sa were synthesized according to Evans method [20], which resulted in dr = 14 1 to 18 1... 

---